The First Hominins

The majority of the evidence of the earliest hominins comes from the Great Rift Valley of East Africa, an expanse that runs from the Horn of Africa at the Red Sea southward to Zambia. During the later Miocene (10 – 5.5 mya) and the early Pliocene (5.5 – 4 mya) at least one lineage of apes made the transition to a more terrestrial and bipedal niche. This shift is thought to have been brought about by climactic changes that were occurring in equatorial Africa at this time. Anatomical adaptations to the pelvis, spinal column, and other body systems followed in suit.

Sahelanthropus tchadensis

S. tchadensis
• Hopwood, 1933
• TM 266-01-060 (Adult Cranium)
• 7 – 6 MYA
• Chad (West Africa)
A French expedition led by Michel Brunet discovered a fossilized skull which the team nicknamed Toumai, “Hope of Life,” in the Djourab Desert of northern Chad in 2001. Sahelanthropus tchadensis, the “Sahel Man from Chad,” expands the range of earliest possible Hominins outside of eastern Africa and is possibly the oldest member of the family Homininae (Stanford). The Toumai fossil consists of a fairly complete skull, mandibular fragments, and isolated teeth. Sahelanthropus contains a mixture of Hominin and apelike characteristics. Hominin characteristics include: large browridge, canine tooth reduction, non-functional C/P3 honing complex, no diastema, and a horizontal nuchal plane. Apelike characteristics include: small brain (320 – 380 cc), U-shaped dental arcade, and thin enamel. The significance of this specimen is still debated given its recent discovery.

Orrorin tugenensis

O. tugenensis
• Pickford & Senut, 2001
• BAR 1000’00 (Adult Mandible)
• 6 MYA
• Kenya
In 2001 paleontologists Martin Pickford and Brigette Senut announced the discovery of “Millennium Man.” The approximately 6 million year old (myo) fossils were found in the Lukeino formation of the Tugen Hills of Kenya and consists of fragmented cranial and postcranial remains, most importantly femoral fragments. The “Hominin from the Tugen Hills” is argued to be a biped based on internal femur anatomy, but this is not a conclusive bipedal indicator. Size of the femoral head and angle of the femoral neck indicate that Orrorin was at least frequently bipedal (Fuentes). Scientists who feel that this is a Hominin point to its thick molar enamel. Others contend that the large canines are much more apelike.

Ardipithecus ramidus

A. ramidus
• White, et al., 1992
• ARA-VP-6/1 (Associated set of adult teeth)
• 4.4 MYA
• Aramis, Ethiopia
In 1992 an international research team uncovered a number of fossils at the Aramis site along the Awash River in northern Ethiopia. Initially these fossils were placed in the genus Australopithecus, but it was noted that that this find was certainly more primitive than other australopithecines and its locomotor pattern was questionable (Stanford). This announcement is based on isolated teeth, cranial fragments indicating a more anteriorly placed foramen magnum, and partial postcrania. These variations along with thinner enamel and less postcanine tooth enlargement led to the reassignment to a new genus and species, Ardipithecus ramidus “The Ground – Living Root Hominin.”